1. Field of The Invention
The present invention relates to multicolor image forming apparatuses for reproducing multicolor images of documents using electrophotographic process.
2. Description of the Related Art
Conventionally, there have been known multicolor image forming apparatuses which reproduce multicolor images by repeating the following image forming process including a step for reading a document image by a document reading means such as a CCD array decomposing it into individual colors to obtain color data, a step for forming a latent image regarding a certain color on a photoconductive body by driving a laser means according to the color data obtained, a step for developing the latent image using toner of the color and a step for transferring a toner image formed onto an intermediate transferring medium.
Also, there have been known multicolor image forming apparatuses which reproduce multicolor images by repeating an image forming process including steps for forming a latent image on a photoconductive body by scanning a document optically, for erasing unnecessary portions of the latent image using by an erasing means such as an LED array, for developing the latent image using a developing unit containing toner of a predetermined color and for transferring a toner image developed (See Japanese patent laid-open publication No. 60-194469).
The latter method has such a great merit that the mechanism forming the multicolor image can be simplified in comparison with the former method using the laser means. However, the pitch of the arrangement of LEDs (the unit of erasing) is not necessarily equal to the pitch of the arrangement of CCDs (the unit of reading). If the unit of erasing is minimized to the order of the unit of reading, the cost of the LED array becomes too high to use it in the multicolor image forming apparatus of this type.
In a conventional multicolor image forming apparatus of the latter type, the pitch of the LED array is designed to have a pitch of multiple times (usuall 3 or 4 times) of the pitch of the CCD array and, accordingly, an area of the unit of erasing defined on the photoconductive body becomes 9 or 16 times of an area of the unit of reading. In other words, 9 or 16 color data obtained by the CCD are included in a single unit of erasing. These 9 or 16 color data are not necessarily same to each other. In such a case that all color data are not same to each other, some of them are lost in the image forming process since the erasing process is performed in the unit of erasing.
In Japanese patent laid open publication No. 60-235170, there is proposed a method for preventing an image to be reproduced from deformation in configurations thereof by controlling individual light amounts of LEDs according to color data in the unit of erasing. This method utilizes the fact that the surface voltage of a latent image formed on the photoconductive body varies according to the brightness of color. Namely, if color data to be erased and not to be erased are contained in a single unit of erasing, the light amount of the LED is adjusted so as to be able to erase the color data to be erased but to maintain the color data not to be erased.
In this method, if red color portion and black color portion coexist in the unit of erasing, it is possible to erase the red color portion since the difference between the brightnesses of red and black colors is large. However, a black toner image is superimposed on a red toner image or magenta or yellow toner image) upon visualizing the black portion of a latent image by black toner and, thereby, the hue of color is changed inevitably. Further, if two colors having the brightnesses not so different from each other coexist in the unit of erasing, it becomes difficult to erase either one of them since the difference between two surface voltages corresponding to individual color portions is too small. This invites changes in configurations of image. Also, if it is possible to erase either one of them in that case mentioned above, it is inevitable for a toner image of a color having a lower brightness to be superimposed on a toner image of another color having a higher brightness. This invites changes in the hue. Since changes in the hue are caused over the whole image to be reproduced, only a poor image having a disharmoniour color balance is reproduced.